The pelleting process has been known for years and generally includes the mixing of dry feed material with moisture and heat to form a mash, and then forcing the mash through the holes in a rotating die. A set of stationary knives is oriented to trim the extruded mash as the die rotates to then form the pellets. Almost universally in the prior art, pelleting systems utilized steam in the mixing or conditioning portion of the process. This is generally considered by those of ordinary skill in the art as necessary in order to fully and completely heat the dry material, add appropriate amounts of moisture, and ensure a complete mixing of the ingredients in the conditioner to form a homogenous mash before it is pelleted. One of the principal advantages of utilizing steam is that the steam changes phase within the conditioner and thereby contributes its heat of vaporization as it condenses into liquid water to thereby provide an added amount of heat to the mash. Heating is important for many processes and formulations as the pellet mill may be operated at quite elevated temperatures. It is not uncommon to operate the pellet mill with mash temperatures approaching and even exceeding 180.degree. F. At elevated throughput rates and low ambient temperatures (such as in the winter), quite significant amounts of heat are required to ensure that the mash temperature is elevated to a proper value. Therefore, virtually every pellet mill installation and process includes the use of steam in the conditioner as a source of both heat and moisture in transforming the dry feed material into a mash suitable for pelleting in the rotating die.
One of the most important parameters utilized to control the pelleting process and also to judge the quality of a finished pellet is the amount of moisture utilized in the pelleting process and the percentage of moisture contained in the finished pellet. The inventor herein is also the inventor of the following patents disclosing and claiming automatic control systems for pellet mills and various features thereof: U.S. Pat. Nos. 3,932,736; 4,340,937; 4,463,430; 4,671,757; 4,678,424; and 4,700,310. The importance of controlling moisture in the pelleting process is emphasized in many of these prior automatic control systems and has been approached from different perspectives. For example, in the earliest patent, U.S. Pat. No. 3,932,736, it was desired to control the moisture content by utilizing temperature measurements taken as the mash traversed the conditioner or input section to the pellet mill. A later patent, U.S. Pat. No. 4,340,937, provided another control mode which utilized a temperature differential taken across the die or pellet producing section of the pellet mill. Still, a later patent, U.S. Pat. No. 4,742,463, provided a control whereby the temperature of the finished pellet immediately as it is formed is taken and is used to control the input of moisture and heat in an effort to achieve finished pellets having a desired moisture content. As can be appreciated, much work has been expended by the inventor herein in developing controllers for pellet mills which desirably control the input of moisture and heat to achieve finished pellets having a desired moisture content. In all of these systems, steam is a major ingredient in the process and is dealt with rather effectively, but in different manners depending upon the desired control mode and source for steam.
Despite the tremendous success which the inventor has experienced in developing and commercializing these prior patented automatic controllers, the inventor has continued his efforts at improving the automation of the pelleting process. One of the problems inherent in utilizing steam in the pelleting process is that it is very difficult to accurately measure the moisture content of the steam. Of course, the moisture content can depend not only upon the temperature of the steam, but also upon the pressure of the steam. Furthermore, it is rather expensive in a typical pellet mill installation to provide a boiler capable of producing the large amounts of steam required as an ingredient in the pelleting process. When steam is used as an ingredient, the system is an open loop system which means that steam does not return to the boiler or other steam source for recycling. Therefore, large amounts of energy are utilized in producing the large quantities of steam required when steam is an ingredient in the pelleting process.
In an effort to partially address the inherent process control problems in pelleting when utilizing steam, there has been developed a steam or vapor generator device which utilizes a burner placed directly in contact with water to produce steam at a desired super heated temperature and desired moisture level on a continuous flow basis. The inventor herein has succeeded in adapting that steam vapor generator for use with a pellet mill and automatically controlling it therewith as shown in U.S. Pat. No. 4,700,310. Still other control schemes for the steam vapor generator when used with a pellet mill are pending. However, the steam vapor generator merely represents a more controllable source for steam and does not alleviate the inherent problems in utilizing steam in the pelleting process. As mentioned above, utilizing steam still represents an inherent loss in control due to the relative difficulty in accurately measuring the moisture content of steam and controlling its production and introduction into the pellet mill. Furthermore, when steam is used as an ingredient, large quantities of steam must be produced for consumption which can be quite expensive not only in the cost of capital equipment but also in the energy costs for producing pellets.
To solve these and other drawbacks inherent in the pelleting process which utilizes steam as an ingredient, the inventor herein has succeeded in developing a new approach to pelleting which utilizes moisture in its liquid phase, i.e. water, and heat in the form of hot air, but no steam in converting the dry material into a mash suitable for pelleting. This represents a complete departure from the prior art where the prevailing thinking is that steam is a necessary ingredient in the pelleting process. Instead of steam, the water is heated to approximately 200.degree. F. and sprayed on the dry material in the conditioner, and air heated to a nominal 275.degree. F. is blown through the conditioner at positive air pressure to heat the mash. The source for the heated air generally comprises a blower mounted in an air duct which also contains a number of heaters such that the blower pressurizes the air through the heaters and into a plenum which interconnects the air duct to the conditioner. For convenience, and as the typical pellet mill installation does include a boiler for other purposes, the heaters in the air ducts may be steam driven heat exchangers, much like radiators found in steam heating systems in commercial building. However, as in typical commercial heating systems, the radiators are in a closed loop system such that after the steam traverses the heat exchanger, it is returned to the boiler and reused. This is much more energy efficient and greatly reduces the required capacity for the boiler over that in a typical prior art pellet mill installation where steam is utilized as an ingredient of the mash itself.
With the pellet mill process as disclosed and claimed herein, the amounts of moisture added to the dry material may be very accurately determined and, when factored appropriately to account for losses, may be very accurately matched to the rate of flow of dry material to produce finished pellets having a desired percentage moisture content. This also provides the advantage of separating the temperature input from moisture input such that they may be individually adjusted independently of each other whereas with steam, an increase in the amount of steam increases both moisture and temperature and vice versa. Therefore, the temperature parameter may be optimized to satisfy other considerations in pelleting, such as die life, or sterilization of the mash, proper cooking temperature, etc.
In some pellet mill installations, it may desirable to continue to utilize steam and rely on the hot air injection system of the present invention as a source of dry heat. As shown in the inventor's prior patent No. 4,671,757, there is a need in pelleting to provide dry heat to the mash to elevate its temperature to ensure recommended cooking or gelatinization of the feed prior to its pelleting. With some materials, such as wet grain or the like, there may already be a significant amount of moisture present such that increasing the temperature by adding steam may be undesirable as it overmoisturizes the feed. Some materials desired to be pelleted contain enough moisture that they may be directly fed into the pellet mill without the addition of any supplemental moisture. In these instances, the moisture supply means can be considered as the same structure which inputs the material to the pellet mill conditioner. In the prior art, steam jackets were often provided surrounding the conditioner and steam was pumped into these jackets to impart heat into the conditioner by radiating heat from the sidewalls thereof. As explained more fully in the prior patent mentioned above, this type of installation suffers from many drawbacks. Therefore, the hot air injection system of the present invention offers a desirable alternative for pumping dry heat into the mash. This dry heat system may thus be used with pellet mill installations incorporating steam, hot water, molasses, and other types of liquids and be thus desirable. However, the inventor believes that optimal advantage of the hot air injection system of the present invention is attained through use thereof without steam in a pellet mill installation for the reasons more fully explained herein.
While the principal advantages and features of the present invention have been described above, a fuller understanding may be obtained by referring to the drawing and description of the preferred embodiment which follow.